Platinum Resistance Thermometer (PRT) Selection GuideSensor Platinum-wire wound or flat-film resistor Thermoelement, two dissimilar metals/alloys Ceramic (metal oxides) Accuracy (typical

Platinum Resistance Thermometer (PRT) Selection Guide

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Thermocouple & Platinum Resistance Thermometry - At A Glance


Comparison of Sensor Types

Platinum Resistance Thermometer Thermocouple Thermistor

Sensor Platinum-wire wound or flat-film resistor

Thermoelement, two dissimilar metals/alloys

Ceramic (metal oxides)

Accuracy (typical values) 0.1 to 1.0°C 0.5 to 5.0°C 0.1 to 1.5°C

Long term Stability Excellent Variable, Prone to ageing Good

Temperature range -200 to 650°C -200 to1750°C -100 to 300°C

Thermal response Wirewound – slow Film – faster 1-50 secs typical

Sheathed – slow Exposed tip – fast 0.1 to 10 secs typical

generally fast 0.05 to 2.5 secs typical

Excitation Constant current required None None

Characteristic PTC resistance Thermovoltage NTC resistance (some are PTC)

Linearity Fairly linear Most types non-linear Exponential

Lead resistance effect 3 & 4 wire – low. 2 wire – high Short cable runs satisfactory Low

Electrical “pick-up” Rarely susceptible susceptible Not susceptible

Interface Bridge 2,3 or 4 wire

Potentiometric input. Cold junction compensation required

2 wire resistance

Vibration effects/ shock wirewound – not suitable. Film – good

Mineral insulated types suitable Suitable

Output/ characteristic approx. 0.4 W/°C From 10µV/°C to to 40µV/°C depending on type

-4% / °C

Extension Leads Copper Compensating cable Copper

Cost Wirewound – more expensive Film – cheaper

Relatively low cost

Inexpensive to moderate

Comments and values shown in this chart are generalised and nominal. They are not intended to be definitive but are stated for general guidance.


RTD Sensor or Thermocouple?

Choosing between a RTD Sensor and a Thermocouple

Resistance Thermometers utilise a high precision sensing resistor, usually platinum, the resistance value of which increases with temperature. The dominant standard adopted internationally is the Pt100 which has a resistance value of 100.0 Ohms at 0°C and a change of 38.50 Ohms between 0 and 100°C (the fundamental interval).

The platinum sensing resistor is highly stable and allows high accuracy temperature sensing. Resistance thermometer sensing resistors are 2 wire devices but the 2 wires will usually be extended in a 3 or 4 wire configuration according to the application, the associated instrumentation and accuracy requirements.

Thermocouples comprise a thermoelement which is a junction of two specifield, dissimilar alloys and a suitable two wire extension lead. The junction is a short circuit only, the EMF is generated in the temperature gradient between the hot junction and the ‘cold’ or reference junction. This characteristic is reasonably stable and repeatable and allows for a family of alternative thermocouple types (e.g. J,K,T,N) to be used.

The alternative types are defined by the nature of the alloys used in the thermoelements and each type displays a different thermal EMF characteristic.

In both cases, the choice of thermocouple or RTD must be made to match the instrumentation and to suit the application.

Thermocouples are, generally:

• Relatively inexpensive• More rugged• Less accurate• More prone to drift• More sensitive• Tip sensing• Available in smaller diameters• Available with a wider temperature range• More versatile

RTD’s are, generally:

• More expensive• More accurate• Highly stable (if used carefully)• Capable of better resolution• Restricted in their range of temperature• Stem, not tip sensitive• Rarely available in small diameters (below 3mm)


Comparison of Sheath Materials

Sheath materials range from mild and stainless steels to refractory oxides (ceramics, so called) and a variety of exotic materials including rare metals. The choice of sheath must take account of operating temperature, media characteristics, durability and other considerations including the material relationship to the type of sensor.

Sheath Material Max Continuous

ture

Notes Applications

Refractory Oxide recrystallised, e.g. Alumina Impervious

1750°C Good choice for rare metal thermocouples. Good resistance to chemical attack. Mechanically strong but severe thermal shock should be avoided.

Forging iron & steel. Incinerators carburizing and hardening in heat treatment. Continuous furnaces. Glass Lehrs.

Silicon Carbide (Porous)

1500°C Good level of protection even in severe conditions. Good resistance to reasonable levels of thermal shock. Mechanically strong when thick wall is specified but becomes brittle when aged. Unsuitable for oxidising atmospheres but resists fluxes.

Forging iron & steel. Incinerators Billet heating, slab heating, butt welding. Soaking pits ceramic dryers.

Impervious Mullite

1600°C Good choice for rare metal thermocouples under severe conditions. Resists Sulphurous and carbonaceous atmospheres. Good resistance to thermal shock should be avoided.

Forging iron & steel. Incinerators. Heat treatment. Glass flues. Continuous furnaces.

Mild Steel (cold drawn seamless)

600°C Good physical protection but prone to rapid corrosion. Annealing up to 500°C. Hardening pre-heaters. Baking ovens.

Stainless steel 25/20 1150°C Resists corrosion even at elevated temperature. Can be used in Sulphurous atmospheres.

Heat treatment annealing, flues, many chemical processes. Vitreous enamelling. Corrosion resistant alternative to mild steel.

Inconel 600/800* 1200°C Nickel-Chromium-Iron alloy which extends the properties of stainless steel 25/20 to higher operating temperatures. Excellent in Sulphur free atmospheres; superior corrosion resistance at higher temperatures. Good mechanical strength.

Annealing, carburizing, hardening. Iron and steel hot blast. Open hearth flue & stack. Waste heat boilers. Billet heating, slab heating. Continuous furnaces. Soaking pits. Cement exit flues & kilns. Vitreous enamelling. Glass flues and checkers. Gas superheaters. Incinerators up to 1000°C. Highly sulphurous atmospheres should be avoided above 800°C.

Chrome Iron 1100°C Suitable for very adverse environments. Good mechanical strength. Resists severely corrosive and sulphurous atmospheres.

Annealing, carburizing, hardening. Iron & steel hot blast. Open hearth flue and stack. Waste heat boilers. Billet heating, slab heating. Continuous furnaces. Soaking pits. Cement exit flues & kilns. Vitreous enamelling. Glass flues and checkers. Gas superheaters. Incinerators up to 1000°C.

Nicrobell* 1300°C Highly stable in vacuum and oxidising atmospheres. Corrosion resistance generally superior to stainless steels. Can be used in Sulphurous atmospheres at reduced temperatures. High operating temperature.

As Inconel plus excellent choice for vacuum furnaces and flues.

* Tradenames


Thin Film Detectors & Wire-Wound Detector Elements

This is just a small selection of Detectors offered by RS / Allied

Image Resistance Dimensions (width x length)

Tolerance Class A Tolerance Class B Tolerance Class 1/3 Din RS order code Allied code RS order code Allied code RS order code Allied code

A Pt100 2 x 5mm 611-7788 70646146 611-7801 70646148 - - A Pt100 2 x 10mm 362-9799 70643577 237-1607 70641762 362-9812 70643578 A Pt100 2 x 2.3mm 362-9834 70643579 362-9840 70643580 362-9856 70643581 A Pt1000 2 x 10mm 362-9907 70643582 362-9913 70643583 814-0178 70656472 A Pt1000 1 x 3mm - - 814-0171 70656470 - - A Pt1000 1.25 x 1.7mm - - 814-0175 70656471 - -

Image Resistance Dimensions (Dia x length)

Tolerance Class A Tolerance Class B Duel Element (Pt100 x2) Tolerance Class A

RS order code Allied code RS order code Allied code RS order code Allied code

B Pt100 1.5 x 8mm 611-7873 70646155 611-7851 70646153 - - B Pt100 1.5 x 15mm 611-7839 70646151 611-7867 70646154 397-1595 70643873 B Pt100 2.8 x 15mm 611-7845 70646152 611-7823 70646150 - - B Pt100 2.8 x 25mm 611-7817 70646149 611-7794 70646147 - -

A B Thin Film Detectors Wire-Wound Detector Elements

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Platinum Resistance Thermometer with Extended Leads

This is just a small selection of Pt100 Sensors offered by RS / Allied

Image Type Class Detector (WxL)

Cable Length

AWG Cable Type Termination RS Order Code

Allied Code

A Pt100 B 2 x 10mm 300mm 24 AWG Teflon® insulated 2 Wire 891-9132 - A Pt100 A 2 x 10mm 1000mm 26 AWG Teflon® insulated 4 Wire 891-9145 - A Pt1000 B 2 x 10mm 500mm 24 AWG Teflon® insulated 2 Wire 891-9157 -

Image Type Class Detector (WxL)

Cable Length

AWG Cable Type Termination RS Order Code

Allied Code

B Pt100 B 2.8 x 15mm 300mm 26 AWG Teflon® insulated 2 Wire 110-4460 - B Pt100 B 2.8 x 15mm 500mm 26 AWG Teflon® insulated 4 Wire 891-9160 - B Pt100 B 2.8 x 15mm 1000mm 26 AWG Teflon® insulated 4 Wire 891-9163 -

Image Type Ceramic Diameter

Ceramic Length

Lead Length Cable Type Termination RS Order Code

Allied Code

C Pt100 5mm 35mm 50mm 7/0.2mm SPC Teflon 2 Wire 237-1641 70641766 C Pt100 5mm 35mm 450mm 7/0.2mm SPC Teflon 4 Wire 237-1657 70641767 C Pt100 5mm 35mm 10mm 1/0.4mm Nickel 4 Wire 237-1629 70641764

A B Platinum Resistance Pt100 & Pt1000 Detectors with Extended Leads

Pt100 Ceramic Wire-Wound C Pt100 Tubular Ceramic Insert Elements with tail wires











Platinum Resistance Pt100 Sensors


Image Type Class Probe Diameter

Probe Length

Cable Length

Cable Type RS Order Code

Allied Code

A Pt100 B 6mm 50mm 2m Flexible silicone rubber insulated, 7/0.2mm 455-3968 70644349 A Pt100 B 6mm 100mm 2m Flexible silicone rubber insulated, 7/0.2mm 611-8264 70646193

Image Type Class Strip Dimensions (LxWxH)

Cable Length


Allied Code

B Pt100 B 35mm x 6mm x 2mm 1m 7/.02mm Teflon® insulated twin twisted lead 237-1613 70641763


Probe Length

Cable Length


Allied Code

C Pt100 B 4mm 150mm 1m 7/0.2mm Teflon® insulated 2 twisted leads 237-1663 70641768

A B Pt100 Heavy Duty Sensor Probe, Class B

Pt100 Sheathed Thin Film Strip Sensor

C PT100 ‘Flat Tip’ Probe






Platinum Resistance Thermometer Pt100 Precision Probes


Image Type Class Probe Length

Cable Length

Cable Type Termination Probe Temperature

Range

RS Order Code

Allied Code

A Pt100 1/5th Din 250mm 2m 7/0.2mm PTFE insulated with silver plated copper screen

4 Wire -50°C to +250°C 236-4299 70641759


Cable Length


Range

RS Order Code

Allied Code

B Mineral Insulated

Duplex PRT

B 150mm 1m 7/0.2mm flexible 6 core Teflon® insulated & screened

2 x 3 wire -50°C to +500°C 397-1416 70643859


Cable Length


Range

RS Order Code

Allied Code

C Pt100 B 150mm 1m 7/0.2mm flexible 6 core Teflon® insulated &

screened

2 x 3 wire -50°C to +250°C 397-1393 70643857

A B Platinum Resistance Thermometer Pt100 Precision Probe

Platinum Resistance Pt100 Dual Element Mineral Insulated Sensor Probe

C Platinum Resistance Pt100 Dual Element Industrial Sensor Probe





Platinum Resistance Thermometer Pt100 with Stainless Steel Sheath



Probe Length

Cable Length

Cable Type Termination RS Order Code

Allied Code

A Pt100 B 3mm 25mm 1m Teflon® insulated 4 Wire 762-1134 70651745 A Pt100 B 3mm 100mm 1m Teflon® insulated 4 Wire 158-985 70636467 A Pt100 B 4mm 90mm 1m Teflon® insulated 4 Wire 123-5610 -


Probe Length

Cable Length


Allied Code

B Pt1000 B 4mm 40mm 1m Teflon® insulated 2 Wire 123-5612 -


Probe Length

Cable Length


Allied Code

C Pt100 B 3mm 150mm 1m Teflon® insulated, Screened 4 Wire 362-9935 - C Pt100 B 4mm 25mm 2m Teflon® insulated, Screened 4 Wire 123-5588 - C Pt100 B 4.5mm 125mm 2m Teflon® insulated, Screened 4 Wire 123-5597 - C Pt100 B 6mm 300mm 2m Teflon® insulated, Screened 4 Wire 123-5606 -

A B Platinum Resistance Pt100 Class B Sensors with Teflon Insulated lead in a Stainless-Steel Tube

Platinum Resistance Pt1000 Class B Sensor with Teflon insulated lead in a Stainless-Steel Tube

C Platinum Resistance Thermometer Pt100 Industrial Sensor Probe, Class B, in a Stainless-Steel Tube










Platinum Resistance Pt100 & Pt1000 Silicone Patch Sensors


Image Type Class Patch Length

Patch Width

Patch Height

Cable Length


Allied Code

A Pt100 B 40mm 13mm 5mm 2m Teflon Insulated, 7/0.2mm

4 Wire 285-661 70637793

A Pt100 B 40mm 13mm 5mm 5m Teflon Insulated, 7/0.2mm

4 Wire 762-1137 70651746

Image Type Class Patch Length

Patch Width

Patch Height

Cable Length


Allied Code

B Pt1000 B 30mm 15mm 4mm 1m Teflon Insulated, 7/0.2mm

2 Wire 762-1130 70651744

A B Pt100 Silicone Patch Sensor Pt1000 Silicone Patch Sensor





Hygienic Platinum Resistance Pt100 Thermometer


Image Sensor Type Sheath Diameter

Sheath Length

Support Diameter

Support Length

Transmitter Fitted (3 Wire Configuration)

No Transmitter Fitted (4 Wire Configuration)

RS Order Code Allied Code RS Order Code Allied Code

A Pt100 6mm 75mm 8mm 50mm 872-2761 - 872-2764 - A Pt100 6mm 125mm 8mm 50mm 872-2770 - 872-2767 -

Image Sensor Type Sheath Diameter

Sheath Length

Support Diameter

Support Length

Transmitter Fitted (3 Wire Configuration)

No Transmitter Fitted (4 Wire Configuration)

RS Order Code Allied Code RS Order Code Allied Code

B Pt100 6mm 75mm 8mm 50mm 872-2786 - 872-2773 - B Pt100 6mm 125mm 8mm 50mm 872-2789 - 872-2777 -

A B Platinum Resistance Pt100 Hygienic Thermometer, 1.5'' Tri-Clamp fitting

Platinum Resistance Pt100 Hygienic Thermometer, 1.5'' RJT style fitting










Pt100 Wall-Mounted Air Boxes


Image Class Length Width Height Indoor/Outdoor Use? RS Order Code Allied Code

A B 85mm 85mm 30mm Indoor Only 338-9491 70643384

Image Class Length Width Height Elements Type

Pt100 Connection

4-20Ma Output

Indoor/Outdoor Use? RS Order Code

Allied Code

B B 80mm 74mm 54mm Single 4 Wire No Indoor or Outdoor use 236-4283 70641758 B B 80mm 74mm 54mm Duplex 2 x 4 Wire No Indoor or Outdoor use 455-4208 70644358 B B 80mm 74mm 54mm Single 3 Wire Yes (2 Wire) Indoor or Outdoor use 455-4214 70644359

Image Cable Glands

Cable Length

Cable Insulation

Transmitter Fitted?

Transmitter Range Indoor/Outdoor Use? RS Order Code

Allied Code

C M16 1000mm PFA Teflon Yes -50°C to +150°C Indoor Only 872-2758 - C M16 1000mm F/G + SSOB Yes 0°C to + 400°C Indoor Only 872-2751 -

A B C Platinum Resistance Pt100 Indoor Air Temperature Sensor

Platinum Resistance Thermometer Pt100 Outdoor/Cold Store Temperature Sensors

4-20mA remote wall mounted housing, Platinum Resistance Pt100 input with 1 metre lead








Platinum Resistance Pt100 with Terminal Heads


Image Sensor Type Probe Diameter Probe Length Head Termination RS Order Code Allied Code

A Pt100 6mm 100 IP67 Din B Head 872-2736 - A Pt100 6mm 200 IP67 Din B Head 872-2733 - A Pt100 6mm 500 IP67 Din B Head 872-2749 -

Image Sensor Type Probe Diameter Probe Length Head Termination RS Order Code Allied Code

B Pt100 6mm 150 IP67 KNS Head 872-2711 - B Pt100 6mm 250 IP67 KNS Head 872-2720 - B Pt100 6mm 300 IP67 KNS Head 872-2727 -

Image Sensor Type

Probe Diameter

Probe Length

Head Termination

Transmitter Fitted?

Transmitter Range RS Order Code Allied Code

C Pt100 6mm 150 IP67 KNE Head Yes -50°C to +150°C 872-2708 - C Pt100 6mm 150 IP67 KNE Head Yes 0°C to 100°C 872-2701 - C Pt100 6mm 150 IP67 KNE Head Yes 0°C to 200°C 872-2705 - C Pt100 6mm 150 IP67 KNE Head Yes 0°C to 400°C 872-2714 -

A B Platinum Resistance Pt100 4 wire class B Resistance Thermometer with Compact KNS Head

Platinum Resistance Pt100 4 wire class B Resistance Thermometer with DIN B Head

C Platinum Resistance Pt100 3 wire class B Resistance Thermometer with KNE Head and Fitted Transmitter












Platinum Resistance Pt100 Industrial Probe with Lagging Extension



Probe Length Below ½” BSPP Process Connection

Lagging Length Termination RS Order Code

Allied Code

A Pt100 B 8mm 250mm 75mm KNE Head 455-3980 70644351


Replaceable insert Length

RS Order Code

Allied Code

B Pt100 B 6mm 275mm 455-4012 70644353

A B 4 wire sensor with 1/2"BSPP process connection

Replaceable insert for 4 wire sensor with 1/2"BSPP process connection




Digital Thermometer / Data Logger

The Labfacility L200 Pt100 thermometer can be used in conjunction with a PC to provide accurate, Pt100 temperature measurement, scanning and logging of measured values. It can also be used as a “stand alone” indicator/logger and incorporates a digital display of measured temperature.

Self-calibration of Pt100 ranges is simple and uses plug-in precision resistors.

The L200 is designed to provide exceptional stability with high measurement resolution and represents an ideal crossover between plant practicality and laboratory performance at a very competitive price.

RS Code Allied Code

910-6826 -

The Labfacility L300 Pt100 temperature alarm / on-off controller can be used in conjunction with a PC to provide accurate monitoring and alarm or on-off control of up to 8- zones simultaneously. It can also be used as a stand-alone instrument without the need for a PC. The PC software supplied with the instrument allows control, configuration, measurement, logging, charting, alarm & relay configuration and calibration functions via a PC. Self-calibration of Pt100 ranges is simple and uses plug-in precision resistors.

RS Code Allied Code

910-6823 -

L200 Digital Thermometer & Data Logger

L300 8-ZONE TEMPERATURE ALARM / ON-OFF CONTROLLER WITH 10A SWITCHING FOR LABORATORY / TRAINING APPLICATIONS




Frequently Asked Questions

Information given here is for general guidance only and is not definitive – it is not intended to be the basis for product installation or decision making. Q. How accurately can I measure temperature using a standard sensor? A. To published, internationally specified tolerances as standard, typically ± 2.5°C for popular thermocouples, ±0.5°C for PRT. Higher accuracy sensors can be supplied to order, e.g. ±0.5°C for type T thermocouple, ±0.2°C for PRT. All of these values are temperature dependent. A close tolerance, 4-wire PRT will give best absolute accuracy and stability. Q. How do I choose between a thermocouple and a PRT? A. Mainly on the basis of required accuracy, probe dimensions, speed of response and the process temperature. Q. What is the difference between a RTD and PRT sensor? A. Nothing. RTD means resistance thermometer detector (the sensing element) and PRT means Platinum resistance thermometer (the whole assembly) i.e. a PRT uses a RTD! Q. What is a Pt100? A. An industry standard Platinum RTD with a value of 100 Ohms @0°C to IEC751; this is used in the vast majority of PRT assemblies in most countries. 12

Q. Are there other types of temperature sensor apart from thermocouple and PRT Types? A. Several, but these two groups are the most common. Alternatives include thermistors, infra-red (non-contact), conventional thermometers (stem & dial types) and many others. Q. Why offer 2,3 or 4 wire PRT probes? A. Because all 3 are encountered. Two-wire should be avoided, three-wire is widely used and four-wire gives optimum accuracy. Your instrument will be configured for 2,3 or 4 wire. Q. What is the minimum immersion depth for a PRT probe? A. Usually 150mm or more; increase the immersion until the reading is unchanged. 12 Q. What is the practical difference between wire-wound and film RTDs? A. Wire-wound type provides greater accuracy and stability but is vulnerable to shock; film type is resistant to shock and has quicker thermal response. Q. Is a sensor with a calibration certificate more accurate than an uncalibrated one? A. No. However, the errors and uncertainties compared with a reference sensor are published and corrected values can be used to obtain better measurement accuracy. Q. How long will my sensor last in the process? A. Not known but predictable in some cases; this will be a function of sensor type, construction, operating conditions and handling.


Frequently Asked Questions

Q. What is the longest thermocouple I can have without losing accuracy? A. Try to ensure a maximum sensor loop resistance of 100 Ohms for thermocouples and 4 wire PRTs. Exceeding 100 Ohms could result in a measurement error. Note By using a 4-20mA transmitter near the sensor, cable runs can be much longer and need only cheaper copper wire. The instrument must be suitable for a 4-20mA input though. 12 Q. Do I need a power supply when using a transmitter, and what length of extension lead can I run with a transmitter fitted? A. A 24Vdc, 20mA supply will be needed if this is not incorporated in the measuring instrument. Long runs of copper cable can be used. Q. What accuracy will I get at a certain temperature using a Pt100 detector; if a better grade detector is used what effect will this have to the accuracy? A. Refer to this Labfacility Temperature Handbook for Pt100 tolerance information. Q. What accuracy loss will I get using a transmitter in line? A. This depends on the accuracy of the specified transmitter; there will always be some degradation. Q. As most instrumentation only takes 2 or 3 wire Pt100s, if I took the correction made on the 3 wire system and incorporated that on to the single leg could I achieve a 4 wire system? A. No; cable length and ambient temperature variations come into play. 12

Q. What is the difference between a flat film and wire wound Pt100 element? A. Film uses platinum deposition on a substrate; wire wound uses a helically wound Pt wire in ceramic. Wire-wound type provides greater accuracy and stability but is vulnerable to shock; film type is resistant to shock and has quicker thermal response. Information given here is for general guidance only and is not definitive – it is not intended to be the basis for product installation or decision making. 12

Documents

Platinum Resistance Thermometer (PRT) Selection GuideSensor Platinum-wire wound or flat-film resistor Thermoelement, two dissimilar metals/alloys Ceramic (metal oxides) Accuracy (typical